1. Field of the Invention
The present disclosure relates to a photomask and a pattern forming method which uses the photomask. More particularly, the present disclosure relates to a photomask which is used for proximity exposure for forming a pattern with an interval smaller than a limit value that can be achieved by exposure in lithography, and a pattern forming method which uses the photomask.
2. Description of the Related Art
In micromachining of a semiconductor integrated circuit, MEMS (micro electro mechanical systems) processing and surface finishing of a flat panel such as liquid crystal, lithography is used in which a photosensitive resin layer formed on a substrate to be processed is subjected to pattern exposure, and the exposed photosensitive resin layer is developed to form a pattern of interest. In lithography, light beams are generally applied through a photomask on which a pattern is drawn. In this case, exposure is usually performed by providing a gap between the photomask and an exposure object, i.e., proximity exposure is usually performed to prevent a damage caused when the photomask closely adheres to the exposure object.
However, when such a gap is provided, even if a pattern of about several multiples of an exposure wavelength dimension is provided on a mask to form a fine pattern of about several multiples of the exposure wavelength dimension, there is a problem that a pattern having a dimension of interest cannot be formed on a substrate which is an exposure object due to the Fresnel diffraction phenomenon or the like. More specifically, even when an exposure light beam whose exposure wavelength is 365 nm is used, if a gap of several tens of μm is provided between a mask and an exposure object, a resolution up to 4 μm to 5 μm can only be obtained.
When exposure is performed with use of exposure photomask 903 in which light shield 902 is formed on transparent substrate 901 as illustrated in, for example, FIG. 48, light beams 904 transmitted through photomask 903 are diffracted, and diffracted light beams 904 blur a pattern before reaching substrate 905 which is an exposure object.
However, in recent years, a projection transfer exposure method is used in some cases, in which a projection lens is provided between photomask 903 and substrate 905 mentioned above. According to the projection transfer exposure method, it is possible to transfer a pattern image to the exposure object and, consequently, a resolution corresponding to an exposure wavelength can be obtained. In this regard, however, a precise lens is necessary, and therefore an exposure device is enormously costly.
Hence, a method is being proposed for achieving a desired pattern of fine dimension in proximity exposure, in which phase shifters which change phases of exposure light beams are provided on a photomask to improve a contrast of light intensities using interferences of light beams.
A method of forming a pattern smaller than a limit value that can be achieved by exposure in lithography with use of photomask 903A will be described with reference to FIG. 49 (see, for example, Patent Literature 1).
As illustrated in FIG. 49, photomask 903A is a phase shift mask in which main pattern (light transmissive portion) 906 including a linear main opening, and transmissive and linear auxiliary patterns (light transmissive portions) 908 provided on a lateral side of main pattern 906 are partitioned by phase shift films 909. Further, light shielding films (chrome films) 907 are provided in a region on an outer side of auxiliary patterns 908.
Auxiliary pattern 908 are each an opening pattern which is not resolved by development processing, and are arranged in regions on both lateral sides symmetrically with respect to a center line of main pattern 906. It is disclosed that phase shift films 909 shift phases of passing light beams by 180° to cause light beams widened by diffraction in main pattern 906 to interfere with light beams of reverse phases transmitted through auxiliary patterns 908 and to cancel each other in order to improve a degraded resolution.